fs/mpage.c

   1 /*
   2  * fs/mpage.c
   3  *
   4  * Copyright (C) 2002, Linus Torvalds.
   5  *
   6  * Contains functions related to preparing and submitting BIOs which contain
   7  * multiple pagecache pages.
   8  *
   9  * 15May2002    Andrew Morton
  10  *              Initial version
  11  * 27Jun2002    axboe@suse.de
  12  *              use bio_add_page() to build bio's just the right size
  13  */
  14
  15 #include <linux/kernel.h>
  16 #include <linux/export.h>
  17 #include <linux/mm.h>
  18 #include <linux/kdev_t.h>
  19 #include <linux/gfp.h>
  20 #include <linux/bio.h>
  21 #include <linux/fs.h>
  22 #include <linux/buffer_head.h>
  23 #include <linux/blkdev.h>
  24 #include <linux/highmem.h>
  25 #include <linux/prefetch.h>
  26 #include <linux/mpage.h>
  27 #include <linux/writeback.h>
  28 #include <linux/backing-dev.h>
  29 #include <linux/pagevec.h>
  30 #include <linux/cleancache.h>
  31 #include "internal.h"
  32
  33 /*
  34  * I/O completion handler for multipage BIOs.
  35  *
  36  * The mpage code never puts partial pages into a BIO (except for end-of-file).
  37  * If a page does not map to a contiguous run of blocks then it simply falls
  38  * back to block_read_full_page().
  39  *
  40  * Why is this?  If a page's completion depends on a number of different BIOs
  41  * which can complete in any order (or at the same time) then determining the
  42  * status of that page is hard.  See end_buffer_async_read() for the details.
  43  * There is no point in duplicating all that complexity.
  44  */
  45 static void mpage_end_io(struct bio *bio)
  46 {
  47         struct bio_vec *bv;
  48         int i;
  49
  50         bio_for_each_segment_all(bv, bio, i) {
  51                 struct page *page = bv->bv_page;
  52                 page_endio(page, bio_data_dir(bio), bio->bi_error);
  53         }
  54
  55         bio_put(bio);
  56 }
  57
  58 static struct bio *mpage_bio_submit(int rw, struct bio *bio)
  59 {
  60         bio->bi_end_io = mpage_end_io;
  61         guard_bio_eod(rw, bio);
  62         submit_bio(rw, bio);
  63         return NULL;
  64 }
  65
  66 static struct bio *
  67 mpage_alloc(struct block_device *bdev,
  68                 sector_t first_sector, int nr_vecs,
  69                 gfp_t gfp_flags)
  70 {
  71         struct bio *bio;
  72
  73         bio = bio_alloc(gfp_flags, nr_vecs);
  74
  75         if (bio == NULL && (current->flags & PF_MEMALLOC)) {
  76                 while (!bio && (nr_vecs /= 2))
  77                         bio = bio_alloc(gfp_flags, nr_vecs);
  78         }
  79
  80         if (bio) {
  81                 bio->bi_bdev = bdev;
  82                 bio->bi_iter.bi_sector = first_sector;
  83         }
  84         return bio;
  85 }
  86
  87 /*
  88  * support function for mpage_readpages.  The fs supplied get_block might
  89  * return an up to date buffer.  This is used to map that buffer into
  90  * the page, which allows readpage to avoid triggering a duplicate call
  91  * to get_block.
  92  *
  93  * The idea is to avoid adding buffers to pages that don't already have
  94  * them.  So when the buffer is up to date and the page size == block size,
  95  * this marks the page up to date instead of adding new buffers.
  96  */
  97 static void
  98 map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
  99 {
 100         struct inode *inode = page->mapping->host;
 101         struct buffer_head *page_bh, *head;
 102         int block = 0;
 103
 104         if (!page_has_buffers(page)) {
 105                 /*
 106                  * don't make any buffers if there is only one buffer on
 107                  * the page and the page just needs to be set up to date
 108                  */
 109                 if (inode->i_blkbits == PAGE_CACHE_SHIFT &&
 110                     buffer_uptodate(bh)) {
 111                         SetPageUptodate(page);
 112                         return;
 113                 }
 114                 create_empty_buffers(page, 1 << inode->i_blkbits, 0);
 115         }
 116         head = page_buffers(page);
 117         page_bh = head;
 118         do {
 119                 if (block == page_block) {
 120                         page_bh->b_state = bh->b_state;
 121                         page_bh->b_bdev = bh->b_bdev;
 122                         page_bh->b_blocknr = bh->b_blocknr;
 123                         break;
 124                 }
 125                 page_bh = page_bh->b_this_page;
 126                 block++;
 127         } while (page_bh != head);
 128 }
 129
 130 /*
 131  * This is the worker routine which does all the work of mapping the disk
 132  * blocks and constructs largest possible bios, submits them for IO if the
 133  * blocks are not contiguous on the disk.
 134  *
 135  * We pass a buffer_head back and forth and use its buffer_mapped() flag to
 136  * represent the validity of its disk mapping and to decide when to do the next
 137  * get_block() call.
 138  */
 139 static struct bio *
 140 do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 141                 sector_t *last_block_in_bio, struct buffer_head *map_bh,
 142                 unsigned long *first_logical_block, get_block_t get_block,
 143                 gfp_t gfp)
 144 {
 145         struct inode *inode = page->mapping->host;
 146         const unsigned blkbits = inode->i_blkbits;
 147         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 148         const unsigned blocksize = 1 << blkbits;
 149         sector_t block_in_file;
 150         sector_t last_block;
 151         sector_t last_block_in_file;
 152         sector_t blocks[MAX_BUF_PER_PAGE];
 153         unsigned page_block;
 154         unsigned first_hole = blocks_per_page;
 155         struct block_device *bdev = NULL;
 156         int length;
 157         int fully_mapped = 1;
 158         unsigned nblocks;
 159         unsigned relative_block;
 160
 161         if (page_has_buffers(page))
 162                 goto confused;
 163
 164         block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 165         last_block = block_in_file + nr_pages * blocks_per_page;
 166         last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
 167         if (last_block > last_block_in_file)
 168                 last_block = last_block_in_file;
 169         page_block = 0;
 170
 171         /*
 172          * Map blocks using the result from the previous get_blocks call first.
 173          */
 174         nblocks = map_bh->b_size >> blkbits;
 175         if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
 176                         block_in_file < (*first_logical_block + nblocks)) {
 177                 unsigned map_offset = block_in_file - *first_logical_block;
 178                 unsigned last = nblocks - map_offset;
 179
 180                 for (relative_block = 0; ; relative_block++) {
 181                         if (relative_block == last) {
 182                                 clear_buffer_mapped(map_bh);
 183                                 break;
 184                         }
 185                         if (page_block == blocks_per_page)
 186                                 break;
 187                         blocks[page_block] = map_bh->b_blocknr + map_offset +
 188                                                 relative_block;
 189                         page_block++;
 190                         block_in_file++;
 191                 }
 192                 bdev = map_bh->b_bdev;
 193         }
 194
 195         /*
 196          * Then do more get_blocks calls until we are done with this page.
 197          */
 198         map_bh->b_page = page;
 199         while (page_block < blocks_per_page) {
 200                 map_bh->b_state = 0;
 201                 map_bh->b_size = 0;
 202
 203                 if (block_in_file < last_block) {
 204                         map_bh->b_size = (last_block-block_in_file) << blkbits;
 205                         if (get_block(inode, block_in_file, map_bh, 0))
 206                                 goto confused;
 207                         *first_logical_block = block_in_file;
 208                 }
 209
 210                 if (!buffer_mapped(map_bh)) {
 211                         fully_mapped = 0;
 212                         if (first_hole == blocks_per_page)
 213                                 first_hole = page_block;
 214                         page_block++;
 215                         block_in_file++;
 216                         continue;
 217                 }
 218
 219                 /* some filesystems will copy data into the page during
 220                  * the get_block call, in which case we don't want to
 221                  * read it again.  map_buffer_to_page copies the data
 222                  * we just collected from get_block into the page's buffers
 223                  * so readpage doesn't have to repeat the get_block call
 224                  */
 225                 if (buffer_uptodate(map_bh)) {
 226                         map_buffer_to_page(page, map_bh, page_block);
 227                         goto confused;
 228                 }
 229
 230                 if (first_hole != blocks_per_page)
 231                         goto confused;          /* hole -> non-hole */
 232
 233                 /* Contiguous blocks? */
 234                 if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
 235                         goto confused;
 236                 nblocks = map_bh->b_size >> blkbits;
 237                 for (relative_block = 0; ; relative_block++) {
 238                         if (relative_block == nblocks) {
 239                                 clear_buffer_mapped(map_bh);
 240                                 break;
 241                         } else if (page_block == blocks_per_page)
 242                                 break;
 243                         blocks[page_block] = map_bh->b_blocknr+relative_block;
 244                         page_block++;
 245                         block_in_file++;
 246                 }
 247                 bdev = map_bh->b_bdev;
 248         }
 249
 250         if (first_hole != blocks_per_page) {
 251                 zero_user_segment(page, first_hole << blkbits, PAGE_CACHE_SIZE);
 252                 if (first_hole == 0) {
 253                         SetPageUptodate(page);
 254                         unlock_page(page);
 255                         goto out;
 256                 }
 257         } else if (fully_mapped) {
 258                 SetPageMappedToDisk(page);
 259         }
 260
 261         if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
 262             cleancache_get_page(page) == 0) {
 263                 SetPageUptodate(page);
 264                 goto confused;
 265         }
 266
 267         /*
 268          * This page will go to BIO.  Do we need to send this BIO off first?
 269          */
 270         if (bio && (*last_block_in_bio != blocks[0] - 1))
 271                 bio = mpage_bio_submit(READ, bio);
 272
 273 alloc_new:
 274         if (bio == NULL) {
 275                 if (first_hole == blocks_per_page) {
 276                         if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
 277                                                                 page))
 278                                 goto out;
 279                 }
 280                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 281                                 min_t(int, nr_pages, BIO_MAX_PAGES), gfp);
 282                 if (bio == NULL)
 283                         goto confused;
 284         }
 285
 286         length = first_hole << blkbits;
 287         if (bio_add_page(bio, page, length, 0) < length) {
 288                 bio = mpage_bio_submit(READ, bio);
 289                 goto alloc_new;
 290         }
 291
 292         relative_block = block_in_file - *first_logical_block;
 293         nblocks = map_bh->b_size >> blkbits;
 294         if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
 295             (first_hole != blocks_per_page))
 296                 bio = mpage_bio_submit(READ, bio);
 297         else
 298                 *last_block_in_bio = blocks[blocks_per_page - 1];
 299 out:
 300         return bio;
 301
 302 confused:
 303         if (bio)
 304                 bio = mpage_bio_submit(READ, bio);
 305         if (!PageUptodate(page))
 306                 block_read_full_page(page, get_block);
 307         else
 308                 unlock_page(page);
 309         goto out;
 310 }
 311
 312 /**
 313  * mpage_readpages - populate an address space with some pages & start reads against them
 314  * @mapping: the address_space
 315  * @pages: The address of a list_head which contains the target pages.  These
 316  *   pages have their ->index populated and are otherwise uninitialised.
 317  *   The page at @pages->prev has the lowest file offset, and reads should be
 318  *   issued in @pages->prev to @pages->next order.
 319  * @nr_pages: The number of pages at *@pages
 320  * @get_block: The filesystem's block mapper function.
 321  *
 322  * This function walks the pages and the blocks within each page, building and
 323  * emitting large BIOs.
 324  *
 325  * If anything unusual happens, such as:
 326  *
 327  * - encountering a page which has buffers
 328  * - encountering a page which has a non-hole after a hole
 329  * - encountering a page with non-contiguous blocks
 330  *
 331  * then this code just gives up and calls the buffer_head-based read function.
 332  * It does handle a page which has holes at the end - that is a common case:
 333  * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
 334  *
 335  * BH_Boundary explanation:
 336  *
 337  * There is a problem.  The mpage read code assembles several pages, gets all
 338  * their disk mappings, and then submits them all.  That's fine, but obtaining
 339  * the disk mappings may require I/O.  Reads of indirect blocks, for example.
 340  *
 341  * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
 342  * submitted in the following order:
 343  *      12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
 344  *
 345  * because the indirect block has to be read to get the mappings of blocks
 346  * 13,14,15,16.  Obviously, this impacts performance.
 347  *
 348  * So what we do it to allow the filesystem's get_block() function to set
 349  * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
 350  * after this one will require I/O against a block which is probably close to
 351  * this one.  So you should push what I/O you have currently accumulated.
 352  *
 353  * This all causes the disk requests to be issued in the correct order.
 354  */
 355 int
 356 mpage_readpages(struct address_space *mapping, struct list_head *pages,
 357                                 unsigned nr_pages, get_block_t get_block)
 358 {
 359         struct bio *bio = NULL;
 360         unsigned page_idx;
 361         sector_t last_block_in_bio = 0;
 362         struct buffer_head map_bh;
 363         unsigned long first_logical_block = 0;
 364         gfp_t gfp = GFP_KERNEL & mapping_gfp_mask(mapping);
 365
 366         map_bh.b_state = 0;
 367         map_bh.b_size = 0;
 368         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 369                 struct page *page = list_entry(pages->prev, struct page, lru);
 370
 371                 prefetchw(&page->flags);
 372                 list_del(&page->lru);
 373                 if (!add_to_page_cache_lru(page, mapping,
 374                                         page->index,
 375                                         gfp)) {
 376                         bio = do_mpage_readpage(bio, page,
 377                                         nr_pages - page_idx,
 378                                         &last_block_in_bio, &map_bh,
 379                                         &first_logical_block,
 380                                         get_block, gfp);
 381                 }
 382                 page_cache_release(page);
 383         }
 384         BUG_ON(!list_empty(pages));
 385         if (bio)
 386                 mpage_bio_submit(READ, bio);
 387         return 0;
 388 }
 389 EXPORT_SYMBOL(mpage_readpages);
 390
 391 /*
 392  * This isn't called much at all
 393  */
 394 int mpage_readpage(struct page *page, get_block_t get_block)
 395 {
 396         struct bio *bio = NULL;
 397         sector_t last_block_in_bio = 0;
 398         struct buffer_head map_bh;
 399         unsigned long first_logical_block = 0;
 400         gfp_t gfp = GFP_KERNEL & mapping_gfp_mask(page->mapping);
 401
 402         map_bh.b_state = 0;
 403         map_bh.b_size = 0;
 404         bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
 405                         &map_bh, &first_logical_block, get_block, gfp);
 406         if (bio)
 407                 mpage_bio_submit(READ, bio);
 408         return 0;
 409 }
 410 EXPORT_SYMBOL(mpage_readpage);
 411
 412 /*
 413  * Writing is not so simple.
 414  *
 415  * If the page has buffers then they will be used for obtaining the disk
 416  * mapping.  We only support pages which are fully mapped-and-dirty, with a
 417  * special case for pages which are unmapped at the end: end-of-file.
 418  *
 419  * If the page has no buffers (preferred) then the page is mapped here.
 420  *
 421  * If all blocks are found to be contiguous then the page can go into the
 422  * BIO.  Otherwise fall back to the mapping's writepage().
 423  *
 424  * FIXME: This code wants an estimate of how many pages are still to be
 425  * written, so it can intelligently allocate a suitably-sized BIO.  For now,
 426  * just allocate full-size (16-page) BIOs.
 427  */
 428
 429 struct mpage_data {
 430         struct bio *bio;
 431         sector_t last_block_in_bio;
 432         get_block_t *get_block;
 433         unsigned use_writepage;
 434 };
 435
 436 /*
 437  * We have our BIO, so we can now mark the buffers clean.  Make
 438  * sure to only clean buffers which we know we'll be writing.
 439  */
 440 static void clean_buffers(struct page *page, unsigned first_unmapped)
 441 {
 442         unsigned buffer_counter = 0;
 443         struct buffer_head *bh, *head;
 444         if (!page_has_buffers(page))
 445                 return;
 446         head = page_buffers(page);
 447         bh = head;
 448
 449         do {
 450                 if (buffer_counter++ == first_unmapped)
 451                         break;
 452                 clear_buffer_dirty(bh);
 453                 bh = bh->b_this_page;
 454         } while (bh != head);
 455
 456         /*
 457          * we cannot drop the bh if the page is not uptodate or a concurrent
 458          * readpage would fail to serialize with the bh and it would read from
 459          * disk before we reach the platter.
 460          */
 461         if (buffer_heads_over_limit && PageUptodate(page))
 462                 try_to_free_buffers(page);
 463 }
 464
 465 static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 466                       void *data)
 467 {
 468         struct mpage_data *mpd = data;
 469         struct bio *bio = mpd->bio;
 470         struct address_space *mapping = page->mapping;
 471         struct inode *inode = page->mapping->host;
 472         const unsigned blkbits = inode->i_blkbits;
 473         unsigned long end_index;
 474         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 475         sector_t last_block;
 476         sector_t block_in_file;
 477         sector_t blocks[MAX_BUF_PER_PAGE];
 478         unsigned page_block;
 479         unsigned first_unmapped = blocks_per_page;
 480         struct block_device *bdev = NULL;
 481         int boundary = 0;
 482         sector_t boundary_block = 0;
 483         struct block_device *boundary_bdev = NULL;
 484         int length;
 485         struct buffer_head map_bh;
 486         loff_t i_size = i_size_read(inode);
 487         int ret = 0;
 488
 489         if (page_has_buffers(page)) {
 490                 struct buffer_head *head = page_buffers(page);
 491                 struct buffer_head *bh = head;
 492
 493                 /* If they're all mapped and dirty, do it */
 494                 page_block = 0;
 495                 do {
 496                         BUG_ON(buffer_locked(bh));
 497                         if (!buffer_mapped(bh)) {
 498                                 /*
 499                                  * unmapped dirty buffers are created by
 500                                  * __set_page_dirty_buffers -> mmapped data
 501                                  */
 502                                 if (buffer_dirty(bh))
 503                                         goto confused;
 504                                 if (first_unmapped == blocks_per_page)
 505                                         first_unmapped = page_block;
 506                                 continue;
 507                         }
 508
 509                         if (first_unmapped != blocks_per_page)
 510                                 goto confused;  /* hole -> non-hole */
 511
 512                         if (!buffer_dirty(bh) || !buffer_uptodate(bh))
 513                                 goto confused;
 514                         if (page_block) {
 515                                 if (bh->b_blocknr != blocks[page_block-1] + 1)
 516                                         goto confused;
 517                         }
 518                         blocks[page_block++] = bh->b_blocknr;
 519                         boundary = buffer_boundary(bh);
 520                         if (boundary) {
 521                                 boundary_block = bh->b_blocknr;
 522                                 boundary_bdev = bh->b_bdev;
 523                         }
 524                         bdev = bh->b_bdev;
 525                 } while ((bh = bh->b_this_page) != head);
 526
 527                 if (first_unmapped)
 528                         goto page_is_mapped;
 529
 530                 /*
 531                  * Page has buffers, but they are all unmapped. The page was
 532                  * created by pagein or read over a hole which was handled by
 533                  * block_read_full_page().  If this address_space is also
 534                  * using mpage_readpages then this can rarely happen.
 535                  */
 536                 goto confused;
 537         }
 538
 539         /*
 540          * The page has no buffers: map it to disk
 541          */
 542         BUG_ON(!PageUptodate(page));
 543         block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 544         last_block = (i_size - 1) >> blkbits;
 545         map_bh.b_page = page;
 546         for (page_block = 0; page_block < blocks_per_page; ) {
 547
 548                 map_bh.b_state = 0;
 549                 map_bh.b_size = 1 << blkbits;
 550                 if (mpd->get_block(inode, block_in_file, &map_bh, 1))
 551                         goto confused;
 552                 if (buffer_new(&map_bh))
 553                         unmap_underlying_metadata(map_bh.b_bdev,
 554                                                 map_bh.b_blocknr);
 555                 if (buffer_boundary(&map_bh)) {
 556                         boundary_block = map_bh.b_blocknr;
 557                         boundary_bdev = map_bh.b_bdev;
 558                 }
 559                 if (page_block) {
 560                         if (map_bh.b_blocknr != blocks[page_block-1] + 1)
 561                                 goto confused;
 562                 }
 563                 blocks[page_block++] = map_bh.b_blocknr;
 564                 boundary = buffer_boundary(&map_bh);
 565                 bdev = map_bh.b_bdev;
 566                 if (block_in_file == last_block)
 567                         break;
 568                 block_in_file++;
 569         }
 570         BUG_ON(page_block == 0);
 571
 572         first_unmapped = page_block;
 573
 574 page_is_mapped:
 575         end_index = i_size >> PAGE_CACHE_SHIFT;
 576         if (page->index >= end_index) {
 577                 /*
 578                  * The page straddles i_size.  It must be zeroed out on each
 579                  * and every writepage invocation because it may be mmapped.
 580                  * "A file is mapped in multiples of the page size.  For a file
 581                  * that is not a multiple of the page size, the remaining memory
 582                  * is zeroed when mapped, and writes to that region are not
 583                  * written out to the file."
 584                  */
 585                 unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
 586
 587                 if (page->index > end_index || !offset)
 588                         goto confused;
 589                 zero_user_segment(page, offset, PAGE_CACHE_SIZE);
 590         }
 591
 592         /*
 593          * This page will go to BIO.  Do we need to send this BIO off first?
 594          */
 595         if (bio && mpd->last_block_in_bio != blocks[0] - 1)
 596                 bio = mpage_bio_submit(WRITE, bio);
 597
 598 alloc_new:
 599         if (bio == NULL) {
 600                 if (first_unmapped == blocks_per_page) {
 601                         if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
 602                                                                 page, wbc)) {
 603                                 clean_buffers(page, first_unmapped);
 604                                 goto out;
 605                         }
 606                 }
 607                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 608                                 BIO_MAX_PAGES, GFP_NOFS|__GFP_HIGH);
 609                 if (bio == NULL)
 610                         goto confused;
 611
 612                 wbc_init_bio(wbc, bio);
 613         }
 614
 615         /*
 616          * Must try to add the page before marking the buffer clean or
 617          * the confused fail path above (OOM) will be very confused when
 618          * it finds all bh marked clean (i.e. it will not write anything)
 619          */
 620         wbc_account_io(wbc, page, PAGE_SIZE);
 621         length = first_unmapped << blkbits;
 622         if (bio_add_page(bio, page, length, 0) < length) {
 623                 bio = mpage_bio_submit(WRITE, bio);
 624                 goto alloc_new;
 625         }
 626
 627         clean_buffers(page, first_unmapped);
 628
 629         BUG_ON(PageWriteback(page));
 630         set_page_writeback(page);
 631         unlock_page(page);
 632         if (boundary || (first_unmapped != blocks_per_page)) {
 633                 bio = mpage_bio_submit(WRITE, bio);
 634                 if (boundary_block) {
 635                         write_boundary_block(boundary_bdev,
 636                                         boundary_block, 1 << blkbits);
 637                 }
 638         } else {
 639                 mpd->last_block_in_bio = blocks[blocks_per_page - 1];
 640         }
 641         goto out;
 642
 643 confused:
 644         if (bio)
 645                 bio = mpage_bio_submit(WRITE, bio);
 646
 647         if (mpd->use_writepage) {
 648                 ret = mapping->a_ops->writepage(page, wbc);
 649         } else {
 650                 ret = -EAGAIN;
 651                 goto out;
 652         }
 653         /*
 654          * The caller has a ref on the inode, so *mapping is stable
 655          */
 656         mapping_set_error(mapping, ret);
 657 out:
 658         mpd->bio = bio;
 659         return ret;
 660 }
 661
 662 /**
 663  * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
 664  * @mapping: address space structure to write
 665  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 666  * @get_block: the filesystem's block mapper function.
 667  *             If this is NULL then use a_ops->writepage.  Otherwise, go
 668  *             direct-to-BIO.
 669  *
 670  * This is a library function, which implements the writepages()
 671  * address_space_operation.
 672  *
 673  * If a page is already under I/O, generic_writepages() skips it, even
 674  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
 675  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
 676  * and msync() need to guarantee that all the data which was dirty at the time
 677  * the call was made get new I/O started against them.  If wbc->sync_mode is
 678  * WB_SYNC_ALL then we were called for data integrity and we must wait for
 679  * existing IO to complete.
 680  */
 681 int
 682 mpage_writepages(struct address_space *mapping,
 683                 struct writeback_control *wbc, get_block_t get_block)
 684 {
 685         struct blk_plug plug;
 686         int ret;
 687
 688         blk_start_plug(&plug);
 689
 690         if (!get_block)
 691                 ret = generic_writepages(mapping, wbc);
 692         else {
 693                 struct mpage_data mpd = {
 694                         .bio = NULL,
 695                         .last_block_in_bio = 0,
 696                         .get_block = get_block,
 697                         .use_writepage = 1,
 698                 };
 699
 700                 ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
 701                 if (mpd.bio)
 702                         mpage_bio_submit(WRITE, mpd.bio);
 703         }
 704         blk_finish_plug(&plug);
 705         return ret;
 706 }
 707 EXPORT_SYMBOL(mpage_writepages);
 708
 709 int mpage_writepage(struct page *page, get_block_t get_block,
 710         struct writeback_control *wbc)
 711 {
 712         struct mpage_data mpd = {
 713                 .bio = NULL,
 714                 .last_block_in_bio = 0,
 715                 .get_block = get_block,
 716                 .use_writepage = 0,
 717         };
 718         int ret = __mpage_writepage(page, wbc, &mpd);
 719         if (mpd.bio)
 720                 mpage_bio_submit(WRITE, mpd.bio);
 721         return ret;
 722 }
 723 EXPORT_SYMBOL(mpage_writepage);